Gradient optimization of polarization exponents inab initio MO calculations on H2SO → HSOH and CH3SH → CH2SH2

Summary Analytical gradients were used to optimize the polarization function exponents in the 6-31G(d) and 6-31G(d, p) basis sets for the reactants, transition structures and products in the reactions H₂SO HSOH and CH₃SH CH₂SH₂. The optimizedd exponents on the heavy atoms change by ±10% in the course of the reactions and depend on the bonding of the heavy atoms. Thep exponents on the hydrogens change by as much as a factor of 5 and depend on the element to which the hydrogen is bonded and its valency. The effect of exponent optimization on the relative energies is small (±3 kcal/mol). With the 6-31G(d, p) basis set, optimization of the polarization exponents can make some of the bonds significantly more polar, as judged by the Mulliken charges.     

Higher order corrections to the processe^ + e^ - \to v\bar vy

A calculation of the neutrino counting reaction \(e^ + e^ - \to v\bar vy\) is presented and implemented in a Monte Carlo generator of unweighted events. The non-photonic corrections are included following the star-scheme. The photonic corrections have been exponentiated following the “inductive” and the “structure functions” approach. For a realistic experimental set-up, the total corrections around theZ ⁰ pole amount to ～5% with respect to the one-loop result. The final precision achieved is better than 1%.     

Evaluation of chiral recognition ability of a novel uranyl-salophen-based receptor: an easy and rapid testing protocol

A novel member of a new class of chiral uranyl-salophen complexes has been synthesised. The chiral recognition ability of this receptor toward the enantiomers of two primary amines, a sulfoxide, and a quaternary ammonium chloride has been evaluated for the first time. The enantioselectivities obtained are encouraging. The NMR method developed for this purpose allows a fast, quantitative determination of the enantioselectivity of the host directly from its racemic mixture and could find application as a preliminary screening tool in the search for new receptors using combinatorial methods. The experiments carried out in this context demonstrated also that the activation barrier for the racemisation of such chiral uranyl-salophen receptors is much higher than the lower limit of 21 kcal mol(-1) previously reported.     

Vibrational Stark effect and vibrational static electric properties of N2O

The vibrational Stark effect together with nuclear relaxation and vibrational contributions to the static molecular electric properties of the N₂O molecule are computed using ab initio molecular orbital thoery. Contributions to the molecular properties are computed by finite-difference techniques involving the energy vs. the uniform electric-field strength. © 1994 John Wiley & Sons, Inc.     

Determination of the temperature dependence of the H-D spin-spin coupling constant and the isotope effect on the proton chemical shift for the compressed dihydride complex [CpIr(P-P)H2]2+

Complex [CpIr(dmpm)H(2)](2+) (dmpm = bis(dimethylphosphino)methane) has been reported to display temperature-dependent spin-spin coupling constant ((1)J(HD)) and isotope effect on the (1)H NMR chemical shift (Deltadelta). A combined electronic structure density functional theory + quantum nuclear dynamics study is used to determine from first-principles the unusual temperature dependence of the spin-spin coupling constant. It is found that the potential energy surface describing the motion of the Ir-H(2) unit has a deeper minimum in the dihydride region and is characterized by important anharmonicities. These anomalies affect the nature of the vibrational states of the unit and are the main reason for the unusual temperature dependence of (1)J(HD) and Deltadelta. These results suggest experimental tests to identify compressed dihydride transition metal complexes.     

Intrinsic reaction coordinate of perturbed potential energy surfaces: Construction of perturbed energy profiles

This contribution analyzes the changes in the intrinsic reaction coordinate (IRC ) and in the potential energy profiles (PEP ) caused by application of a uniform electric field evaluated using standard ab initio MO methods. Two typical organic processes have been studied, namely, the Friedel–Crafts and the Walden inversion reactions, which are modeled by suitable simple systems. The results for the Walden inversion reaction show that the IRC for the field-free and that for perturbed processes are almost coincident; in this case, using the field-free IRC to compute the perturbed energy profile is a very good approximation. On the contrary, for the Friedel–Crafts model reaction, the two IRC s differ slightly, so the energy profiles using the perturbed and the unperturbed IRC s are somewhat different. © 1993 John Wiley & Sons, Inc.     

Structurally simple, modular amino alcohols for the recognition of carboxylic acids. Application to the development of a new chiral solvating agent

[structure: see text] Two flexible receptors for carboxylic acids, based on 1-amino-3-fluoro-2-alcohol functional arrays and built on aminomethylpyridine platforms, are described. The C(2)-symmetric one [from 2,6-bis(aminomethyl)pyridine] has been shown to be an efficient CSA due to its ability to form geometrically different diastereomeric complexes enabling the discrimination between the enantiomers of a series of carboxylic acid in the (1)H NMR spectra.     

Calculation of exchange coupling constants in solid state transition metal compounds using localized atomic orbital basis sets

The application of theoretical methods based on density functional theory using hybrid functionals and localized, atomic orbital type basis sets is shown to provide good estimates for exchange coupling constants in non-metallic, solid state transition metal compounds with relatively complex crystal structures. The accuracy of the calculated exchange coupling constants is similar to that previously obtained for dinuclear and polynuclear molecular compounds. As an application of this procedure, the magnetic properties of the high-temperature phase of CuGeO₃, the recently synthesized silver copper oxide Ag₂Cu₂O₃, and the family of M[N(CN)₂]₂ (M=Cr(II), Mn(II), Fe(II), Co(II), Ni(II) and Cu(II)) compounds are analyzed via the computation of their most relevant exchange coupling constants.     

New camphor-derived sulfur chiral controllers: Synthesis of (2R-exo)-10-methylthio-2-bornanethiol and (2R-exo)-2,10-bis(methylthio)bornane

An efficient preparation of two camphor-derived controllers [(2R-exo)-10-methylthio-2-bornanethiol1b and (2R-exo)-2,10-bis(methylthio)bornane 2] potentially useful as a ligands or chiral auxiliaries in asymmetric synthesis is described. Both compounds have been prepared starting from (1S)-camphor-10-thiol 3. Alkylation of this thiol with sodium methoxide and methyl iodide afforded 10-methylthiocamphor 8. Conversion of 8 into the corresponding thioketone 9 with the Lawesson's reagent followed by the stereoselective reduction with DIBAL-H at low temperature yielded (2R-exo)-10-methylthio-2-bornanethiol1b in good yield. (2R-exo)-2,10-bis(Methylthio)bornane 2 could be obtained by alkylation of 1b with sodium methoxide and methyl iodide.     

Chronoamperometric and Voltammetric Characterization of Gold Ultramicroelectrode Arrays

Some prototypes of photolithographic‐built arrays of gold ultramicroelectrodes (UMEAs) have been electrochemically characterized and compared with a normal‐size electrode. The achievement of steady‐state conditions, effects of supporting electrolyte and the role of conditioning and activation have been considered. The results obtained show that it is possible to monitor the effective area of those devices by chronoamperometry, which confirms the convenience of using the internal auxiliary and reference electrodes and this evidences that steady‐state currents are holding in a wide range of scan rates. Additionally, some preliminary tests show the promising features of gold UMEAs in flow systems like FIA or HPLC.